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Tension/Tent Roofs
Tension or ‘tent’ roof systems, though uncommon in use today outside of large span commercial architecture, offer one of the simplest, toughest, most modular forms of roofing possible at present. Their only practical limitations are their limited production due to often advanced materials processing, generally unusual appearance, and a lack of sound and thermal insulation, though that insulation issue is ameliorated by their use as ‘skybreak’ or detached roofs with insulation provided by other materials. Though traditional tent materials can be used in smaller structures, permanent tension roofs will commonly employ a single large prefabricated membrane or sheet of advanced materials such as Texlon (for transparent roofs or skylights integral to other roof materials), teflon-impregnated fiberglass fabric called PTFE (for translucent roofs with optional insulation), and polyester reinforced vinyl. (for opaque roofs with insulation, but generally inadvisable for use with residential structures) Texlon and PTFE roofs in particular will generally have as long a duty life as any other element of a structure and recent innovations in the use of polyester reinforced aerogels now afford PTFE materials a much higher integral insulation potential than ever before while still maintaining translucency. Tension roofs are governed by the physics of surface tension structures and so their shapes tend to conform to non-euclidean topologies. This may be one of the reasons why they are not more commonly used, as the engineering of these curving tensioned shapes is non-intuitive. But at a relatively small scale they can be quite simple, as we see with common tent shelters. Tension roofs typically use four basic forms; arches, domes, conics, and hypoids. The latter two are most common in commercial architecture use as they use the minimum physical elements to span the largest areas, relying entirely on simple masts tensioning the roof membrane. Conic forms will employ standing center masts to support their ‘peaks’ much like the classic circus tent or ‘floating masts’ which are suspended off the ground by being tensioned on cables -as with a tensegrity truss- between the roof membrane and a perimeter tension ring or frame. Hypoid forms are tensioned between corner masts or arches which tension the membrane in alternating peaks and valleys. With Utilihab, the common forms of tension roof employed are non-tensioned sheds, arched sheds, groin vault frames, and tent hip roof shapes. The non-tensioned shed roof was discussed in the section on shed roofs and simply uses a conventional and simple beam rafter roof structure with a minimum of rafter elements to support a roof membrane that attaches to the rafters using integral snap-in wear strips plugging into the top profile slot of the rafters. This is a sufficient roof structure for small, light, and temporary structures but the roof membrane is only under light tension -or even allowed to sag between the rafters- and so does not integrate well with the main structure at the perimeter of the roof and thus is usually used only in a detached roof arrangement with the roof frame lofted as a whole structure above the main building structure on extended posts. The arched shed employs shallow arch rafters or trusses much as with the supported arch roof to tension the roof membrane tightly into a series of alternating peaks and valleys with the membrane anchored at the perimeter and tensioned by cables across the valleys. A perimeter anchor frame is added to the top of the basic building structure. This can be as simple as an added profile surface plate along the perimeter of upper frame members creating just a small overhang or a full perimeter covered walkway extension frame as used with the standard deck system creating a large overhang supported by its own corner posts. A rounded corner profile may be used in this anchor frame to aid in shedding water or a conventional gutter fascia may be added around its outer edge. The rafters or ‘tension ribs’ attach by tension plugs or angles to the inner side slots of the anchor frame profiles so they rest close to flush with the top of the anchor frame profiles. They may additionally be anchored to main structure upper framing with angles or riser blocks. In some truss designs the upper chord may extend as the roof overhang to the anchor frame while the lower chord attaches to the top of the main structure wall beams. These rafters must have a rounded top edge to allow the membrane to slide freely and may take the form of curved round profiles, trusses with a rounded curved upper chord, or bowstring cable trusses which employ rigid compression members between tension chord cables turnbuckle anchored to the frame or a lower cable and upper anchor points bonded to the roof membrane. The roof membrane attaches by a series bolt eyes in a reinforced edge to pre-installed anchor bolts in the upper profile slot of the anchor frame. These are spaced using a holed sealing strip along the profile slot which serves as a compression gasket. Where there are wide eaves extending over the main structure, membrane may be pre-fitted with a soffit panel or screen which attaches in this same fashion to the upper outer facing profile slots of the top frame members of the main structure. This panel would feature pre-installed holes to allow the truss or rafters to pass through, requiring they be installed after the initial fitting of the membrane. To tension the roof membrane, capstan blocks/modules are attached to the outside edge of the anchor frame and tension cables are strung over the roof membrane with a swages and turnbuckles. The turnbuckles are then used to tighten the cables, pulling downward on the valleys of the membrane and tensioning it between the cables and the tops of the arch rafters. In some cases, a higher arch shape may be used to allow rafters with a sharper end curve to mount on top of the anchor frame rather than flush with it, allowing the roof membrane to be attached in the same but along the outer side profile slots rather than the top profile slot. This provides better roof run-off. The groin vault frame works largely the same as arched shed but uses curved and crossed arch members reminiscent of a groin vault. This is used to support a more steeply curved roof shape using fewer tension cables. The structure is largely the same as with the arched shed except that the groin frame supports are assembled as four legged units that attach on top of the anchor frame or the main structure. The fitted roof sheet then attaches by the same bolt arrangement to the outer sides of the frame profiles. Gutters/facia will typically be used to conceal these series of bolts The same tension cables and capstans blocks are employed to mount tension cables between the groin frame units. The tent hip roof relies on a conic tent form that is reminiscent of a conventional hip roof. The main structure supports a peak anchor for the top of the roof which takes the form of a circle, ovoid, or a rounded end rectangle which is also used to host a peak vent or optional molded acrylic skylight. This is supported over the main structure either by a light version of the gabled roof framing supporting a ridge beam to which this peak anchor attaches or extended Primary posts where peak anchor rests over the line between two structural bays. (this being the simpler arrangement) The tent roof is sized to just clear the corners of the top of the main structure when tensioned, an optional integral soffit ‘skirt’ or screen bolting to the upper outer perimeter edge profile slots of the Primary frame. The roof membrane is pulled over the structure and the peak attached to the peak anchor by eyelets and quick-links to bolted hoist rings on the peak anchor profiles. The small opening here is covered by a lashed-down ridge vent cover or flashing around any possible skylight. The corners of the tent are then tensioned on masts around the base of the building -and usually attached to either their own pier foundation footings or sharing those footings with a perimeter deck system. Turnbuckle tensioners on the piers allow for tensioning of the roof membrane. Though relatively simple, this form of tension roof is limited to structures of fairly uniform rectilinear layout and is most commonly employed in small cottage structures. much more research is needed for this class of roof systems. Available literature on the subject is a bit scarce and assistance from established commercial tension roof companies like Birdair may be needed